Final answer:
The main difference between Protestant and Catholic beliefs is that Protestants believe in salvation through faith alone, whereas Catholics believe salvation is achieved through a combination of faith and good works.
Step-by-step explanation:
The primary difference between Protestant and Catholic beliefs lies in the concept of how salvation is achieved. Protestants, particularly those following Martin Luther's teachings, believe that faith alone is sufficient for salvation. This is supported by Luther's interpretation of the Bible, specifically St. Paul’s epistle to the Romans which states "The just shall live by faith" (Romans 1:17), implying that God's grace cannot be earned by good works, but is a gift freely given to those with faith.
In contrast, the Catholic Church maintains that salvation is a combination of faith and good works. This includes not just personal acts of charity and piety, but also formal practices such as buying indulgences, which were regarded by the Church as good works supporting the religious institution. Moreover, Catholicism upholds the authority of the Pope and a series of traditions that have developed over centuries within the Church.
To answer the initial question, option B is correct: Protestants believe good works alone cannot gain you salvation. Unlike option A which falsifies Protestant beliefs, option C which inaccurately represents Catholic views on papal authority, and option D which incorrectly suggests Catholics allow priests to marry while in fact, it's Protestant denominations that do so, reflecting the changes initiated during the Reformation by figures such as Martin Luther.